1. Field of the Invention
The invention relates to optical waveguide devices and systems employing wavelength division multiplexing. More particularly, the invention relates to mode discriminating coupling of optical signals within such optical devices and systems.
2. Description of the Related Art
Wavelength-division-multiplexed (WDM) optical fiber systems are known systems characterized by the simultaneous transmission of many different communication channels over different wavelengths within a single optical fiber. For example, communication channels are transmitted over wavelengths typically within the 1530-1565 nanometer (nm) range, and separated by multiples of 100 gigahertz (GHz), i.e., approximately 0.8 nm. Within such systems, the ability to efficiently separate and detect the different wavelengths of light traveling through a single fiber is extremely advantageous. For example, it is desirable to have within WDM optical fiber systems a spectrally-selective device that monitors the different channels and key optical parameters (such as the presence or absence of channels and optical signal-to-noise ratios) at the add/drop nodes or at the optical amplifier locations. Also, for example, within field-installed WDM optical fiber systems, the ability to monitor the optical powers in individual channels with, e.g., a hand-held power measurement device, is extremely useful for obvious reasons.
Existing techniques and devices are known and considered in attempting to address the needs mentioned above. For example, U.S. Pat. No. 5,450,512 issued Sep. 12, 1995 to Asakura discusses a means of detecting the combined optical power of all channels and sending it through free space where it is incident upon a diffractive element such as a bulk grating, which separates the individual channels. These spatially separated channels are then recombined into a fiber end through graded index lenses for power monitoring or processing. The process of bringing light out of the fiber, separating it via a bulk-optic component such as a diffractive grating and then recombining it has several disadvantages. For example, alignment and assembly costs are high and time consuming, the long-term reliability is suspect and insertion losses are prohibitive. In general, it is considered prudent to keep the light within the fiber until it is to be incident upon a detector, such as a detector array. The use of a detector array is disclosed in detail, e.g., in co-pending application Ser. No. 08/741,439 filed Oct. 31, 1996 (now U.S. Pat. No. 5,832,156) and assigned to the assignee of the present invention. The use of a detector array, however, adds additional cost and difficulties such as those associated with accurate alignment and assembly.
Therefore, it is desirable to have available a wavelength separator that uses an in-fiber device and does not invoke the need for additional expensive components such as detector arrays.